In the manufacture of metal strip it is usual to reduce a hot metal workpiece down to strip by passing it, while hot, through the stands of a rolling mill. There are usually at least six stands arranged in tandem and the hot workpiece is progressively reduced in thickness as it is passed through the stands. To ensure that the strip, assuming it to be of steel, has the required metallurgical, properties, it is necessary for the temperature of the strip as it passes through the last stand, to be of the order of 900.degree. C. In a conventional hot strip mill with a delay table between the roughing and finishing mills there is a considerable temperature differential between head and tail ends of the workpiece as it enters the first stand of the finishing train and care has to be taken to ensure that the temperature of the strip along its length is at or close to the required temperature as the strip passes through the last stand.
Furthermore, the mill has to be operated at a relatively slower speed, known as the threading speed, as the head of the workpiece passes through the stands and is attached to a coiler located downstream of the last stand, and thereafter the speed of the mill is increased to its rolling speed. Some mills are accelerated at a constant rate as the strip is passed therethrough, while others are accelerated in a succession of bursts each followed by a period of constant speed. In both methods, the average acceleration rate may be of the order of 0.06 m/s.sup.2 for a mill having six stands. Attempts have been made to increase this acceleration rate in order to increase the throughput of the mill but these attempts have so far met with little success because, as the speed is increased, the temperature of the strip at the last stand in the train rises and, if the speed increase is too great, the temperature rises above an acceptable level.
An effect of the temperature differential referred to above is that the rolling loads on the stands increase as the strip is passed through the mill. This variation in rolling load is not equal on each of the stands and consequently the relationship between the rolling loads on the various stands which is set during threading, is not maintained as the strip is accelerated through the mill. This has an undesirable effect on the shape of the strip produced in the mill and particularly strip produced from the tail end of the workpiece may have a shape which is unacceptable to the user.
It is known to provide Vee jet water sprays from nozzles located between the first two or three stands in the mill for the purpose of suppressing the growth of scale on the surface of the workpiece. These sprays are known as "scrubber sprays". Such sprays can only apply a uniform supply of water to the surface of the strip to cool it uniformly if (1) all the jets are correctly set, (2) all the jets are functioning correctly, and (3) the strip is positioned at a preset distance from the spray nozzles. Conditions (1) and (2) are seldom achieved for more than a short time after each maintenance period and the third condition can never be met in practice because the position of the strip relative to the nozzles is continuously changing due to the effect of loopers which are positioned between the stands. The loopers raise and lower the level of the strip in response to the tension in the strip and this is continuously changing. These known scrubber sprays have not resulted in uniform cooling of the strip material and some of the strip produced with mills having scrubber sprays is unacceptable because of striping of the strip with non-uniform temperature zones.
It is known to use these scrubber sprays and possibly further sprays located between the later stands in the mill to increase the acceleration rate by progressively switching them on as the mill speed increases but this can make the normal increase in rolling load greater with further harmful effects on strip quality.